1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium, more particularly to a reversible thermosensitive recording medium comprising a reversible thermosensitive recording layer of which transparency or color tone is reversibly changeable depending upon the temperature thereof, thereby recording information therein and erasing recorded information therefrom repeatedly as desired. The reversible thermosensitive recording may be used in information recording devices in any form, for instance, in the form of a card, a disk, a label, a disk cartridge or a tape cassette. The present invention also relates to a method of producing the above reversible thermosensitive recording medium. The present invention also relates to a method of recording images and erasing the same, using the reversible thermosensitive recording medium.
2. Discussion of Background
Recently attention has been paid to a reversible thermosensitive recording material capable of temporarily recording images therein and erasing the same therefrom when such images become unnecessary. For example, Japanese Laid-Open Patent Application 55-154198 discloses a representative example of a reversible thermosensitive recording material in which an organic low-molecular-wieght material such as a higher fatty acid is dispersed in a matrix resin such as a vinyl chloridexe2x80x94vinyl acetate copolymer.
However, such a conventional reversible thermosensitive recording material has a shortcoming that a temperature range in which the recording material exhibits light transmission or transparency characteristics or is in a transparent stare (hereinafter referred to as the transparentizing temperature width) is as narrow as 2 to 4xc2x0 C., so that it is difficult to control the temperature for performing such image formation while utilizing the properties of reversibly becoming transparent or light shielding or opaque or milky white.
With this shortcoming of the above reversible thermosensitive recording material taken into consideration, the inventors of the present invention previously proposed to facilitate image erasure (making recorded images transparent) by using a mixture of a higher fatty acid and an aliphatic dicarboxylic acid to broaden the transparentizing temperature width to about 20xc2x0 C. as described in Japanese Laid-Open Patent Applications 2-1363 and 3-2089.
In order to improve such erasability, it is also proposed to broaden the transparentizing temperature width by using a mixture of (a) a higher ketone or a fatty acid ester having a lower melting point than that of the higher fatty acid, and (b) an aliphatic dicarboxylic acid or a saturated aliphatic bisamide as described in Japanese Laid-Open Patent Applications 4-366682, 5-294062 and 6-255247. These methods are capable of broadening the transparentizing temperature width and accordingly capable of improving the erasability. However, due to the use of the higher ketone or fatty acid ester having a lower melting point than that of the higher fatty acid, the transparentizing temperature width is situated in a low temperature range, so that these methods have a shortcoming that the formed opaque or milky white images are erased when the ambient temperature thereof is high, for instance, when placed on a dashboard in midsummer.
It has been proposed to shift the transparentizing temperature width to a high temperature side by using a mixture of (a) an organic low-molecular-weight compound having a low melting point and (b) an alicyclic dicarboxylic acid having a melting point of about 200xc2x0 C. which is significantly higher than the melting points of aliphatic dicarboxylic acids (as described in Japanese Laid-Open Patent Applications 5-139053, 6-48024 and 6-48025, or by using a mixture of (a) an organic low-molecular-weight compound having a low melting point and (bxe2x80x2) a low-molecular compound having a steroid skeleton having a melting point near 200xc2x0 C. (as described in Japanese Laid-Open Patent Applications 8-20167 and 8-282131). However, despite the use of the above-mentioned compounds having such extremely high melting points, the upper-limit of the transparentizing temperature width is not shifted so much to a high temperature side and accordingly the erasability is not substantially improved. In order to improve the erasability, there is no choice but to use an organic low-molecular-weight compound having a low melting point, but when an organic low-molecular-weight compound having a low melting point is used, the heat resistance of images formed tends to be lowered.
Furthermore, the recording media disclosed in the above-mentioned Japanese laid-open patent applications have the shortcomings that the temperature difference between a transparentizing upper-limit temperature and an opaqueness initiation lower-limit temperature is so large that a significantly large amount of energy is required for the formation of milky white images, and that the durability of the media is lowered while in repeated use, with the surface of the recording media scratched, and the opaqueness of the image lowered in the course of repeated image printing and erasure.
When a large amount of energy is required for the image formation, a thermal head""s pulse application time is required to be lengthened since there is a limit to a voltage that can be applied to the thermal head from a power source, or the recording speed has to be lowered. Furthermore, when the recording speed is lowered, or when the amount of energy applied to the thermal head is increased, the life of the thermal head is shortened. Thus, when the amount of energy required for the image formation is increased, the applied energy has adverse effects on an apparatus using the reversible thermosensitive recording medium. In this case, it is considered that the high opaqueness initiation temperature is caused by the use of the low-molecular weight compound having an excessively higher melting point.
Furthermore, there are proposed reversible thermosensitive recording media using a leuco dye and a color developer having a long chain alkyl group, capable of inducing a color in the leuco dye in Japanese Laid-Open Patent Applications 5-124360, 5-294063 and 6-171225. The recording media have an advantage over other conventional recording media that images can be developed in a variety of colors such as black and red, when the dye therefor is appropriately selected for use in the recording media, but have a shortcoming that the heat resistance of the images is inferior to that of image s obtained by the above-mentioned reversible thermosensitive recording media capable of forming milky white images with a transparent background or transparent images with a milky white background. It is considered that this shortcoming is caused by the vibrations of the long chain alkyl group of the color developer, initiated when heated to 50 to 60xc2x0 C. even though the melting point of the color developer is as high as 150xc2x0 C. or more, which vibrations cause the color developer to separate from the dye, and accordingly decolorize the images.
It is therefore a first object of the present invention to provide a reversible thermosensitive recording medium with an extended transparentizing temperature width, while maintaining the capability of producing images with high heat resistance, and with high repeated use durability, which is capable of producing images with high contrast and erasing the same with high erasability even when the ambient temperature is high.
A second object of the present invention is to provide a method of producing the above reversible thermosensitive recording medium.
A third object of the present invention is to provide an information recording device utilizing the reversible thermosensitive recording medium of the present invention.
A fourth object of the present invention is to provide a method of recording images in any of the reversible thermosensitive recording medium of the present invention and the information recording devices and/or erasing recorded images therefrom.
The first object of the present invention can be achieved by a reversible thermosensitive recording medium capable of assuming a first color in a first temperature range which is above room temperature, having a first color assuming initiation temperature and a first color assuming upper-limit temperature, and assuming a second color when heated to a second temperature range which is above the first temperature range and then cooled, having a second color assuming lower-limit temperature,
(a) the first color assuming upper-limit temperature being 125xc2x0 C. or more,
(b) the difference between the first color assuming upper-limit temperature and the second color assuming lower-limit temperature being 20xc2x0 C. or less, and
(c) the first color assuming initiation temperature being 95xc2x0 C. or more.
The above reversible thermosensitive recording medium may further have a first color assuming lower-limit temperature, and it is preferable that a first color variation temperature width which corresponds to the difference between the first color assuming upper-limit temperature and the first color assuming lower-limit temperature be 20xc2x0 C. or more.
It is also preferable that the first color assuming upper-limit temperature is 130xc2x0 C. or more in the above reversible thermosensitive recording medium.
It is also preferable that the difference between the first color assuming upper-limit temperature and the second color assuming lower-limit temperature be 15xc2x0 C. or less in the above reversible thermosensitive recording medium.
The first object of the present invention can also be achieved by a reversible thermosensitive recording medium comprising a reversible thermosensitive recording layer which comprises a matrix resin and an organic low-molecular-weight material dispersed in the matrix resin, of which transparency is reversibly changeable depending upon the temperature thereof, the organic low-molecular-weight material comprising a mixture of at least one straight chain compound (A) comprising at least one bond selected from the group consisting of amide bond, urea bond and sulfonyl bond, and at least one carboxyl group, and having a melting point of 130xc2x0 C. or more, and at least one straight chain compound (B) having a melting point which is lower by at least 20xc2x0 C. than the melting point of the straight chain compound (A), the reversible thermosensitive recording medium capable of assuming a first color at a first temperature range which is above room temperature, having a first color assuming initiation temperature and a first color assuming upper-limit temperature, and assuming a second color when heated to a second temperature range which is above the first temperature and then cooled, having a second color assuming lower-limit temperature,
(a) the first color assuming upper-limit temperature being 125xc2x0 C. or more,
(b) the difference between the first color assuming upper-limit temperature and the second color assuming lower-limit temperature being 20xc2x0 C. or less, and
(c) the first color assuming initiation temperature being 95xc2x0 C. or more.
The above reversible thermosensitive recording medium may further have a first color assuming lower-limit temperature, and it is preferable that a first color variation temperature width which corresponds to the difference between the first color assuming upper-limit temperature and the first color assuming lower-limit temperature be 20xc2x0 C. or more.
It is also preferable that the first color assuming upper-limit temperature be 130xc2x0 C. or more in the above reversible thermosensitive recording medium.
In the above reversible thermosensitive recording medium, it is also preferable that the difference between the first color assuming upper-limit temperature and the second color assuming lower-limit temperature be 15xc2x0 C. or less.
In the above reversible thermosensitive recording medium, it is preferable that the straight chain compound (B) have a melting point of 85xc2x0 C. or more.
In the above reversible thermosensitive recording medium, it is also preferable that the straight chain compound (B) have a melting point of 120xc2x0 C. or less.
In the above reversible thermosensitive recording medium, it is also preferable that the straight chain compound (B) have a melting point in a range of 85xc2x0 C. to 120xc2x0 C.
In the above reversible thermosensitive recording medium, it is preferable that the straight chain compound (B) and the straight chain compound (A) be mixed in a mixing ratio by parts by weight of 98:2 to 10:90.
In the above reversible thermosensitive recording medium, it is preferable that as the straight chain compound (A), a straight chain compound comprising an amide bond and a carboxyl group be used.
Furthermore, it is preferable that as the straight chain compound (A), a straight chain compound of general formula (1) be used:
HOOCxe2x80x94(CH2)n-Xxe2x80x94(CH2)m-Yxe2x80x94(CH2)n-COOHxe2x80x83xe2x80x83(1)
wherein 1xe2x89xa6nxe2x89xa626, 1xe2x89xa6mxe2x89xa626, and X and Y each independently represent CONH or NHCO, but do not have an identical structure at the same time.
In the above reversible thermosensitive recording medium, it is preferable that as the straight chain compound (A), a straight chain compound comprising a urea bond and a carboxyl group be used. For example, it is preferable that a straight chain compound of general formula (2) be used:
CH3xe2x80x94(CH2)n-Zxe2x80x94(CH2)m-COOHxe2x80x83xe2x80x83(2)
wherein 0xe2x89xa6nxe2x89xa625, 1xe2x89xa6mxe2x89xa626, and Z represents NHCONH.
The straight chain compound (A) may be a straight chain compound comprising a sulfonyl bond and a carboxyl group, for example, a straight chain compound of general formula (3):
CH3xe2x80x94(CH2)n-Zxe2x80x94(CH2)m-COOHxe2x80x83xe2x80x83(3)
wherein 0xe2x89xa6nxe2x89xa625, 1xe2x89xa6mxe2x89xa626, and Z represents SO2.
It is also preferable that in the above reversible thermosensitive recording medium, the organic low-molecular-weight material further comprise at least one straight chain compound (C) in the mixture, having a melting point which is higher by at least 10xc2x0 C. than that of the straight chain compound (B) and is lower by at least 10xc2x0 C. than that of the straight chain compound (A).
The second object of the present invention can be achieved by a method of producing a reversible thermosensitive recording medium comprising a support, and a reversible thermosensitive recording layer formed thereon comprising a matrix resin and an organic low-molecular-weight material dispersed in the matrix resin, of which transparency is reversibly changeable depending upon the temperature thereof, capable of assuming a first color in a first temperature range which is above room temperature, having a first color assuming initiation temperature and a first color assuming upper-limit temperature, and assuming a second color when heated to a second temperature range which is above the first temperature and then cooled, having a second color assuming lower-limit temperature,
(a) the first color assuming upper-limit temperature being 125xc2x0 C. or more,
(b) the difference between the first color assuming upper-limit temperature and the second color assuming lower-limit temperature being 20xc2x0 C. or less, and
(c) the first color assuming initiation temperature being 95xc2x0 C. or more, comprising the steps of:
coating on the support a dispersion comprising a solvent, the matrix resin and the organic low-molecular-weight material comprising an organic low-molecular-weight compound having a melting point of 130xc2x0 C. or more, which organic low-molecular-weight material is dispersed in the form of a solid in the matrix resin, and
drying the dispersion with application of heat thereto so as to dissolve the organic low-molecular-weight material in the solvent when heat is applied thereto, thereby forming the reversible thermosensitive recording layer on the support.
In the above method, it is preferable that the organic low-molecular-weight material dispersed in the dispersion have a solubility of 0.5% or more in the solvent at a temperature at which the dispersion coated on the support is dried with application of heat thereto.
It is also preferable that in the above-mentioned method, the organic low-molecular-weight material dispersed in the dispersion have a solubility of less than 0.5% in the solvent at room temperature.
The second object of the present invention can also be achieved by a method of producing a reversible thermosensitive recording medium comprising a support, and a reversible thermosensitive recording layer formed thereon comprising a matrix resin and an organic low-molecular-weight material dispersed in the matrix resin, of which transparency is reversibly changeable depending upon the temperature thereof, capable of assuming a first color at a first temperature which is above room temperature, having a first color assuming initiation temperature and a first color assuming upper-limit temperature, and assuming a second color when heated to a second temperature which is above the first temperature and then cooled, having a second color assuming lower-limit temperature,
(a) the first color assuming upper-limit temperature being 125xc2x0 C. or more,
(b) the difference between the first color assuming upper-limit temperature and the second color assuming lower-limit temperature being 20xc2x0 C. or less, and
(c) the first color assuming initiation temperature being 95xc2x0 C. or more, comprising the steps of:
coating on the support a dispersion comprising a solvent, the matrix resin and the organic low-molecular-weight material comprising (a) at least one organic low-molecular-weight compound and (b) an organic low-molecular-weight compound having a melting point of 130xc2x0 C. or more, which organic low-molecular-weight material is dispersed in the form of a solid in the matrix resin, and
drying the dispersion with application of heat thereto at a temperature which is lower than the highest melting point of the melting points of the organic low-molecular-weight compounds, and then at a temperature which is not lower than the highest melting point of the melting points of the organic low-molecular-weight compounds, thereby forming the reversible thermosensitive recording layer on the support.
The third object of the present invention can be achieved by a card comprising a reversible thermosensitive recording portion which comprises any of the above-mentioned reversible thermosensitive recording media, and an information memory portion.
In the above-mentioned card, the information memory portion may comprise at least one element selected from the group consisting of a magnetic recording layer, IC and an optical memory.
It is preferable that the above-mentioned card further comprise a support on one side of which the reversible thermosensitive recording portion is provided, and that the information memory portion comprise the magnetic recording layer and be provided on a back side of the support opposite to the reversible thermosensitive recording portion with respect to the support.
The third object of the present invention can also be achieved by a reversible thermosensitive recording label comprising a support, a reversible thermosensitive recording portion which comprises any of the above-mentioned reversible thermosensitive recording media, which is provided on one side of the support, and an adhesive or tacky layer on a back side of the support opposite to the reversible thermosensitive recording portion.
The third object of the present invention can also be achieved by a disk cartridge comprising a cartridge, a rewritable disk in which information to be recorded therein is rewritable, which rewritable disk is built in the cartridge, and a reversible thermosensitive display portion which comprises any of the above-mentioned reversible thermosensitive recording media, which reversible thermosensitive recording display is provided on the surface of the cartridge.
The third object of the present invention can also be achieved by a disk cartridge comprising a cartridge, a rewritable disk in which information to be recorded therein is rewritable, which rewritable disk is built in the cartridge, and the above-mentioned reversible thermosensitive recording label, which reversible thermosensitive recording label is provided on the surface of the cartridge.
The third object of the present invention can also be achieved by a disk comprising a rewritable or write-once disk in which information to be recorded therein is rewritable or writable, and a reversible thermosensitive display portion which comprises any of the above-mentioned reversible thermosensitive recording media, which reversible thermosensitive display portion is provided on the surface of the rewritable or write-once disk.
The third object of the present invention can also be achieved by a disk comprising a rewritable or write-once disk in which information to be recorded therein is rewritable or writable, and the above-mentioned reversible thermosensitive recording label, which reversible thermosensitive recording label is provided on the surface of the rewritable or write-once disk.
The third object of the present invention can also be achieved by a tape cassette comprising a cassette member, a rewritable tape member in which information to be recorded therein is rewritable, disposed in the cassette member, and a reversible thermosensitive display portion which comprises any of the above-mentioned reversible thermosensitive recording medium, which reversible thermosensitive display portion is provided on the surface of the cassette member.
The third object of the present invention can also be achieved by a tape cassette comprising a cassette member, a rewritable tape member in which information to be recorded therein is rewritable, disposed in the cassette member, and the above-mentioned reversible thermosensitive recording label, which reversible thermosensitive recording label is provided on the surface of the cassette member.
Any of the above-mentioned reversible thermosensitive recording medium, the card, the reversible thermosensitive recording label, the disk cartridge, the disk, and the tape cassette may further comprise a portion in which an image can be irreversibly printed, or which includes such irreversibly printed image.
The fourth object of the present invention can be achieved by a method of recording images or erasing recorded images comprising applying heat for recording or erasing images to one of recording media selected from the group consisting of the above-mentioned reversible thermosensitive recording medium, the above-mentioned card, the above-mentioned reversible thermosensitive recording label, the above-mentioned disk cartridge, the above-mentioned disk, and the above-mentioned tape cassette.
In the above-mentioned method, the application of heat for recording images and/or for erasing recorded images may be carried out, using a thermal head.
Furthermore, in the above-mentioned method, the erasing of recorded images may be carried out by overwriting, with the application of heat thereto, and then images may be recorded, using a thermal head.
For the application of heat for erasing recorded images, a ceramic heater may be used. It is preferable that the ceramic heater be set at a temperature of 110xc2x0 C. or more for the application of heat for erasing recorded images.